Somatic hypermutation is the targeted process of mutagenesis which introduces single base changes into the rearranged variable regions encoding immunoglobulin heavy and light chains. Somatic hypermutation results in the production of B cell clones with affinity for antigen increased 10-fold or even 50-fold, thus enhancing the efficiency of the immune response. The rate of somatic hypermutation is one single base change per 1000 bases per generation, which is nearly one million-fold higher than the typical rate of mutation in a mammalian somatic cell. Mutations are targeted to the V regions of the rearranged heavy and light chain loci, and do not appear in constant regions, in unrearranged V regions, or elsewhere in the genome. Our long-term goal is to understand in detail the molecular mechanism and regulation of immunoglobulin gene hypermutation. Our specific experimental plans are (1) to identify regulatory elements that target hypermutation of a rearranged transgene in cis, by testing the ability of specific sequences to target mutation of a rearranged light chain gene in transgenic mice; (2) to determine what role gene conversion plays in somatic hypermutation, by assaying hypermutation of engineered substrates in transgenic mice; (3) to identify and study factors that may regulate somatic hypermutation, by searching for DNA binding proteins that are specific to or specifically induced in hypermutating B cells, and that recognize sequence elements we have shown to be critical for hypermutation; and (4) to identify genes that are actively transcribed in hypermutating B cells. These experiments address the mechanism of a fundamental process in the immune response. Furthermore, since the somatic events that diversify the cellular genome during B lymphocyte development probably occur by directed use of mechanisms common to all cells, understanding the mechanism of somatic hypermutation of the immunoglobulin loci is likely to provide insight into analogous mutation events that are central to genomic evolution, genetic disease, and oncogenesis.